Using Bash to concatenate the first 10 bytes of many files-CodePudding

I am trying to write a simple Bash loop to concatenate the first 10 bytes of all the files in a directory. So far, I have the code block:

for filename in /content/*.bin; 
do
  cat -- (`head --bytes 10 $filename`) > "file$i.combined"
done

However, the the syntax is clearly incorrect here. I know the inner command:

head --bytes 10 $filename

...returns what I need; the first 10 bytes of the passed filename. And when I use:

cat -- $filename > "file$i.combined"

...the code works, only it concats the entire file contents.

How can I combine the two functions so that my loop concatenates the first 10 bytes of all the looped files?

CodePudding user response：

The loop will do the concatenation for you (or rather, the output of the sequential executions of head are written in order to the standard output of the loop itself); you don't need to use cat.

for filename in /content/*.bin; do
    head --bytes 10 "$filename"
done > "file$i.combined"

CodePudding user response：

With head from GNU coreutils:

head -qc 10 /content/*.bin > combined

CodePudding user response：

You should use @chepner's answer, but to show how you could use `cat`...

Here we have a recursive function that rolls everything up into a single copy of cat.

catHeads() {
  local first="$1"; shift || return
  case $# in
    0) head --bytes 10 "$first" ;;
    1) cat <(head --bytes 10 "$first") <(head --bytes 10 "$1");;
    *) cat <(head --bytes 10 "$first") <(catHeads "$@");;
  esac
}

catHeads /content/*.bin >"file$i.combined"

Don't ever actually do this. Use chepner's answer (or Socowi's) instead.

You should use @chepner's answer, but to show how you could use cat...

You should use @chepner's answer, but to show how you could use `cat`...